It is well known that in certain transportation systems, such as in modern high speed railway and mass and/or rapid transit operations, there is a heed for providing improved wheel slip detection and control apparatus which will more safely and efficiently slow down and/or stop the vehicles or a train at a station, at the entrance of a signal block or the like. A slip condition occurs when more braking force is exerted on the wheel axle unit than that which can be sustained by the available amount of frictional adhesion that exists between the tread of the wheels and the running surface of the rail. A slipping effect causes the wheel axle unit to decelerate at a higher rate than the vehicle deceleration. The primary object of wheel slip control is to reduce the braking force to a point where it is less than the equivalent force of the available adhesion. In practice, such a brake force reduction will attempt to stop the wheel axle unit from decelerating faster than the vehicle and will cause the wheel axle unit to accelerate back up to the speed of the vehicle.
One prior art wheel slip control system is shown and disclosed in U.S. Pat. No. 4,491,920 issued on Jan. 1, 1985, entitled "Rate Polarity Shift Wheel-Slip Control System" which is assigned to the assignee of this invention and which is incorporated by reference to the present application.
Briefly, the wheel-slip control system is for a multiple-truck vehicle including a speed sensor for generating signals representative of the velocity of each of the wheel axle units. A differentiator is connected to each of the speed sensors for differentiating the velocity signals to obtain rate signals. A rate-determining circuit determines the most negative-going rate signal of each of the wheel axle unit of each truck. A plurality of deceleration threshold and rate direction detectors and data processing logic initiate a brake force reduction action on the truck experiencing a wheel slip, and a positive logic "OR" gate senses a polarity shift of the most negative rate signal to cause the data processing logic to reapply a braking action.
Another prior art wheel slip control system is shown and disclosed in U.S. Pat. No. 4,941,099, issued on Jul. 10, 1990, entitled "Electronic Adhesion Adaptive Wheel Slide Arrangement function," which is assigned to the assignee of this invention and which is incorporated by reference to the subject application.
Briefly, the electronic slip control system is for a multiple-truck railway vehicle, including a standard slip control logic circuit which is responsive to axle speed and rate signals to produce a multi-bit binary number word. A primary slip control word formation circuit is used for converting the multi-bit binary number word to a hexidecimal number word which is conveyed to a primary slip control table. The primary slip control table conveys the hexidecimal number word to a table output selection circuit. A synchronous slip control logic circuit is responsive to the axle speed and rate signals to produce a multi-bit binary number word. A synchronous slip control word formation circuit converts the multibit binary number word to a hexidecimal number word which is conveyed to a synchronous slip control table. The synchronous lip control table conveys the hexidecimal number word to the table output selection circuit which causes an intermediate circuit to convey an appropriate control signal to a brake valve for preventing wheel slip.
Each of the above wheel slip control processes is related in the fact that they both use an end of wheel slip sequence speed reference. However, it has been found that during situations in which all of the wheels on a vehicle are experiencing slippage, the utilization of the highest speed wheel on the vehicle for establishing a speed reference is highly unreliable from the standpoint of signal accuracy. It will be appreciated that wheel slip controllers which use an end of wheel slip sequence speed referencing process to control wheel slip effectively conclude each wheel slip correction sequence by causing the slipping wheel to be brought back up to the speed of the vehicle. This form of wheel slip control dramatically improves the accuracy of the speed reference signal which is based on the simple technique of selecting the highest speed wheel on a multiple wheel vehicle. It will be recognized that wheel slip controllers which are specifically designed for providing maximum wheel protection and for minimizing the vehicular stopping distance. It has been found that the usage of the highest speed wheel of the vehicle is a very effective method which produces fairly accurate results. However, this is not the case in which the wheel slip controller is set up to minimize braking force modulations since the controller tends to maximize the sequence time period. Accordingly, this will result in relatively long periods of time to correct all the slipping wheels which have significant levels of slippage.